The process of gravel packing to restrict and control the passage of particulate materials from a subterranean formation into a well which penetrates the formation is well-known. The process of gravel packing basically comprises a mixing of a selected size-grading of gravel, sand or other particulate material in a fluid carrier and pumping the resulting slurry down the annulus between the wellbore wall or well casing and a centrally located well screen or perforated liner. The fluid carrier filters through the screen and is returned to the surface leaving a porous permeable pack of granular material in the wellbore annulus between the wall of the wellbore and the well screen. This porous pack substantially reduces the migration of particles from the formation into the well as well as reducing the associated problems of equipment erosion and wellbore plugging when fluids containing such formation particles are produced.
In addition to forming a granular filter pack in the wellbore annulus, some of the gravel and carrier fluid travels through casing perforations into the formation itself. While such technique effectively enlarges the gravel pack with its attendant advantages, formation damage can result from large amounts of fluid carrier material being "filtered out" into the formation. Additionally, high pressure gravel packing can cause actual formation breakdown and rapid fluid loss in the gravel pack slurry which will result in sand bridging at a particular location. Bridging can effectively reduce or totally stop the overall gravel packing process in other locations in the well. This is particularly true when there is a low formation pressure counteracting the hydrostatic pressure of a conventional packing process.
Gravel packing is typically carried out in accordance with a so-called slurry-pack process such as is described in U.S. Pat. No. 3,498,380. In accordance with the slurry-pack process, an aqueous carrier fluid is thickened with a gelling agent such as a natural or synthetic gum, polysaacharide, polyacrylamide polymers, cellulose derivatives and the like. This thickened fluid is capable of carrying up to twenty pounds of particulate material such as sand per gallon of fluid. Typically, sand loadings of ten to fifteen pounds per gallon are used. With sand loadings of this magnitude, the hydrostatic pressure created on the formation in even a moderately deep well can be substantial. If formation pressures are substantially exceeded by the hydrostatic pressure, breakdown of the formation and rapid loss of the gravel pack fluid material under the formation can occur with the results being an incomplete gravel pack due to sand-out or bridging as well as the possibility for substantial formation damage with the influx of large amounts of fluid carrier material.
The complications involved in a slurry pack process at low formation pressures are even further aggravated by a completion involving several producing zones over a long interval such as may be typical in a gas well wherein several formation layers produce into a single wellbore annulus having a continuous gravel pack. Formation breakdown and resultant bridging or sand-out in one stratum can result in failure to complete the remaining portions of the gravel packing of the entire wellbore.
Other methods of completing a long interval are costly, time consuming and complicated either by high hydrostatic pressures or difficult and complicated completion methods. The well-known pre-pack/sand injector-type gravel pack can occassionaly be used over a long interval but formations having low formation pressure eliminate this process since the foam clean-out of pre-packed sand using a nitrogen foam at high pressures can cause formation damage under such low formation pressure conditions. Further, there is a significant cost in this process due to the additional equipment, time and manpower required over simplier processes. Additionally, it is difficult to build a significant sand reservoir above the screen top since the sand injection process contains only relatively low sand loadings throughout the process.
The final, most complex method for completion of gravel pack operations for several producing formations over a long interval would be a process whereby individual zones were isolated with a packer and, upon, completion of the gravel packing of such zone, other packers are then set across other intervals for additional gravel packing and continuing this process until the entire interval is completed. The complications involved in multiple packer settings, precise control of sand injection pressures over several operations and the strong possibility that a single failed packing operation could destroy the effectiveness of the entire gravel pack are apparent.
In a typical gravel pack operation, an excess of particulate material is packed into the wellbore annulus above the screen and perforations to act as a reservoir of additional particulate material which can settle into the screen/perforation area if a void occurs in the initial gravel pack or as the result of gravel loss through the screen during production. Again, a relatively high sand loading in the fluid carrier material in order to build up this reservoir results in high hydrostatic pressures which could result in the aforementioned problems of formation breakdown, fluid loss and bridging. Low sand loadings require an extended pumping period to build a sufficient release of sand for the reservoir.
Under normal conditions, a formation is typically acidized prior to the placement of a gravel pack. The acid penetrates the formation dissolving soluble particulate materials and enlarging fluid passages so that formation fluids are more easily produced. However, under low bottomhole pressure conditions, prior acidizing can aggravate the chances of formation breakdown during gravel packing with the resulting problems associated therewith.